Thermistors are used in a variety of applications to measure the temperature of materials. For example, thermistors are used to measure the temperature of various components in an internal combustion engine as well as other components in an automobile such as coolant lines in an air conditioning system.
To measure the temperature of the material with the thermistor, it is typically necessary to place the thermistor in intimate contact with the material whose temperature is being measured. To obtain intimate contact between the thermistor and the material, it is frequently necessary to form an aperture in the structure that holds the material. The aperture also typically permits the thermistor to be removably attached to the structure. To prevent escape of the material from the structure, an impervious seal must be formed between the thermistor and the structure as well as between the components in the thermistor.
One technique for preventing the escape of material through the components of the thermistor involves placing a cover over an end of the thermistor that extends into the structure. For example, Metzger et al., U.S. Pat. No. 5,046,857, describes preventing the material whose temperature is being measured from passing through the thermistor by placing a thermistor pill within an outer shell. Temperature of a material outside the outer shell is measured with the thermistor pill positioned in the outer shell. Similarly, Clayton, Jr., U.S. Pat. No. 4,437,084, discloses a thermistor in which a thermistor pill is encapsulated in an outer shell.
A drawback of these encapsulating techniques is that the accuracy of the temperature measurement is limited because the thermistor pill is not in direct contact with the material whose temperature is being measured. As such, temperature changes must be transmitted through the material that encompasses the thermistor pill.
A prior art technique for manufacturing a thermistor 300 that does not require the use of an outer shell uses a threaded nut 302 having a central bore 304, as most clearly illustrated in FIG. 4. A thermistor pill assembly 306 is retained in a fixed relation to the threaded nut 302 with a sealing glass 308, which bonds to both the threaded nut 302 and conductive leads 310 extending from a thermistor pill 312. The high temperature necessary to prepare the glass seal 308 typically causes the conductive leads 310 to weaken because of annealing. After the glass seal 308 is formed, it is necessary to plate the threaded nut 302 to reduce degradation of the threaded nut 302 during use.
Another technique for fabricating a thermistor that is not encapsulated is disclosed in Baier, U.S. Pat. No. 4,987,749. Baier indicates that the conductive wires that extend from the thermistor pill are mounted in a glass header. The header glass is soldered to the wall of the thermistor assembly to prevent refrigerant fluid from passing between the components of the thermistor.